Fuel cells continue to show great commercial promise throughout the world as an alternative to conventional energy sources. This commercial promise should continue to grow as energy shortages become more acute, environmental regulations become more stringent, and new fuel cell applications emerge. See, e.g.,"FUEL CELLS", Encyclopedia of Chemical Technology, 4th Ed., Vol. 11, pp. 1098-1121. Strikingly, numerous automotive manufacturers have announced and continue to announce plans for mass production and retail sale of fuel cell-powered cars in the near future.
Despite improvements in fuel cell technology, however, long felt needs generally exist to increase power output, reduce initial cost, improve water management, and lengthen operational lifetime. Initial cost reduction, which is particularly important, can be most easily achieved by reducing the precious metal content of the fuel cell electrode. Such reduction, however, generally results in power output loss which blocks commercialization efforts. Hence, new discoveries are needed to resolve these and other difficult compromises.
There are different types of fuel cells, but they each produce electrical energy by means of chemical reaction. One type of increasing import, the "polymer electrolyte membrane fuel cell" (PEMFC), comprises a membrane electrode assembly (MEA) typically made of an ionically conducting polymeric membrane sandwiched between two electronically conducting electrodes. The electrodes, besides being conductive, are also associated with electrocatalyst layers which provide catalysis. For commercial application, multiple MEAs can be electronically connected to form a fuel cell stack (i.e., "stacked"). Other components associated with typical PEMFCs include gas diffusion media and current collectors, the latter of which can also serve as bipolar separators and flow field elements. PEMFCs have been reviewed in the literature.
See S. Srinivasan et al.; J. Power Sources; 29 (1990); pp. 367-387; and Fuel Cell Systems, L. J. M. J. Blomen and M. N. Mugerwa (Ed.); Plenum Press; 1993; Chapter 11.
In a typical PEMFC, a fuel such as hydrogen gas is electrocatalytically oxidized at one electrode (anode). At the other electrode (cathode), an oxidizer such as oxygen gas is electrocatalytically reduced. The net reaction, when mediated by the membrane, results in generation of electromotive force and external current flow between the electrodes. Elevated temperature can accelerate this reaction, although one increasingly important advantage of the PEMFC is that lower temperatures (e.g., 80.degree. C.) can be used. The fuel cell reactions are generally catalyzed by precious transition metals, commonly a noble metal such as platinum, which are present in both anode and cathode as an electrocatalyst layer. Because the fuel cell is often operated with use of gaseous reactants, typical electrodes are porous materials (or more generally, reactant diffusive materials) having the catalytically active metal at or within the porous surfaces. In some cases, catalyst may be thinly coated by polymer which allows gas to diffuse to the underlying catalyst. The metal can be in different morphological forms, but often it is in particulate or dispersed form and supported on carbon. Fuel cell performance can depend on the catalyst form. See Poirier et al.; J. Electrochemical Society, vol. 141, no. 2, February 1994, pp. 425-430.
Fuel cell systems are complex because the reaction is believed localized at a three-phase boundary between ionically conducting membrane, gas, and carbon supported catalyst. Because of this localization, addition of ionically conductive material to the electrode can result in better utilization of catalyst as well as improved interfacial contact with the membrane. However, the additional ionic conductor can introduce extra cost, especially when perfluorinated conductors are used, and can increase the complexity of electrolyte water management, all important to commercialization. Hence, discoveries are needed which improve utilization of ionic conductor.
To minimize the loading of expensive catalytic metal, one general approach has been to use smaller catalyst particles. However, long operational lifetimes are particularly difficult to achieve with lower catalyst loadings, and catalyst poisoning can occur. Also, catalyst particle size may be unstable and increase by agglomeration or sintering. Hence, discoveries are also needed to improve utilization of catalyst, and the combination of catalyst and ionic conductor.
Another generic approach which has been tried without success has been to concentrate the metal at the membrane-electrode interface. See, e.g., Ticianelli et al.; Journal of Electroanalytical Chemistry and Interfacial Electrochemistry; Vol. 251 No. 2, Sep. 23, 1988, pp. 275-295. For example, 500 angstrom dense layers of single metal catalyst reportedly have been sputtered onto certain gas diffusion electrodes before sandwiching the ionically conducting membrane between the electrodes. However, sputtered layers thinner than 500 angstroms were not reported, possibly because of the difficulty in making uniform thinner layers. Moreover, the concentration of catalyst approach may not be suitable for other types of electrodes and deposition techniques and may upset water balance. Further, testing often is not carried out under commercial conditions. Particularly poor performance was reported for electrodes in which all of the catalyst metal was in the form of a sputtered film. In sum, it is recognized that mere vapor depositing an allegedly thin layer of catalyst onto the electrode does not guarantee a suitable MEA for commercial applications, and in general, industry has not accepted this approach as realistic. . According to the Srinivasan article noted above, for example, sputtering may not be economically feasible compared with wet chemical deposition methods.
Additional technology is described in the patent literature including, for example, U.S. Pat. Nos. 3,274,029; 3,492,163; 3,615,948; 3,730,774; 4,160,856; 4,547,437; 4,686,158; 4,738,904; 4,826,741; 4,876,115; 4,937,152; 5,151,334; 5,208,112; 5,234,777; 5,338,430; 5,340,665; 5,500,292; 5,509,189; 5,624,718; 5,686,199; and 5,795,672. In addition, deposition technology is described in, for example, U.S. Pat. Nos. 4,931,152; 5,068,126; 5,192,523; and 5,296,274.
Although much research has focused on fuel cell electrodes, particularly significant developments relating to fuel cell membranes are described in U.S. Pat. Nos. 5,547,551; 5,599,614; and 5,635,041 (Bahar et al.). For commercial applications, membrane design should be integrated with electrode design in a systemic approach to maximize fuel cell performance. Combinations of properties, which are vital for commercialization, can be difficult to achieve without this integrated approach.
Finally, another problem which can arise and block commercialization is catalyst poisoning which is caused by impurities such as carbon monoxide (CO) in the reactants. For example, when hydrogen fuel is generated by hydrocarbon reforming, CO can be co-generated which is expensive to remove, particularly when the CO level in hydrogen is reduced to below 100 ppm. Poisoning is especially problematic in PEMFCs which have low catalyst loadings and which employ the single metal platinum. Discoveries are needed to solve poisoning problems without generating other problems and to provide suitable compromises.
Although attempts to mitigate CO poisoning have been reported, they generally have been unsuccessful and have resulted in reduced power. Some of these attempts have focused on mixing platinum with other transition metals such as ruthenium before deposition on the electrode. Deposition methods have included, for example, wet ink and vacuum methods. However, ink methods can be difficult to control precisely, and some vacuum methods can be expensive and cumbersome, particularly for thin film deposition.
Methods of using transition metal catalyst mixtures are discussed in, for example, U.S. Pat. Nos. 4,430,391; 4,487,818; 5,296,274; 5,395,704; and 5,786,026. In addition, Morita et al. describe a gold-platinum bimetallic model catalyst on smooth carbon support by RF sputtering. See "Anodic Oxidation of Methanol at a Gold-Modified Platinum Electrocatalyst Prepared By RF Sputtering on a Glassy Carbon Support,"; Electrochimica Acta, vol. 36, No. 5/6, pp. 947-951 (1991). However, this article reports problems in obtaining consistent results and reproducible data with bimetallic systems. In addition, this article only describes methanol oxidation which is different mechanistically from other fuel oxidations. It does not describe hydrogen oxidation and the use of reactant diffusive (or porous) electrodes. The thicknesses of the catalytic zones are not reported although thicknesses less than one micron are noted. Alloys are not described, and deposition methods other than RF sputtering are not described.
In addition, U.S. Pat. No. 5,750,013 describes a membrane electrode assembly based on a vacuum-deposited ionically conducting membrane which is positioned between vacuum-deposited alternating layers of microparticle metal layer and porous conducting layer. The entire membrane electrode assembly is prepared by vacuum deposition. However, there is no concentration of the catalytically active metal at the membrane-electrode interface. Rather, the layered structures described are not concentrated and would be expected to have relatively poor catalytic efficiency. In addition, a well-integrated three-phase boundary between the ionic conductor of the membrane, the electronic conductor of the electrode, and the catalytically active metal of the electrode is not present. Moreover, the vacuum-deposited membrane is excessively thin and would not generally be a suitable fuel cell barrier in practical applications. Finally, no experiments are reported in this patent on the performance of the membrane electrode assembly, particularly under commercially realistic conditions.
U.S. Pat. Nos. 4,430,391 and 4,487,818 (Ovshinsky et al.) describe fuel cell electrodes in which a host matrix, which comprises transition metal, is modified with at least one modifier element to improve catalytic properties. The modified catalytic layer can be deposited onto a catalyst-free gas diffusion electrode by sputtering. The minimum thickness of the catalytic layer is described as 0.5 microns (5,000 angstroms). According to these patents, the modifier element increases the amount of disorder in the host matrix which increases the number of catalytically active sites in the electrode. Although experimental data are reported in these patents, no experimental data are reported for a working membrane electrode assembly or fuel cell. Also, polymer electrolyte membrane fuel cells are not taught or suggested.
U.S. Pat. Nos. 5,879,827 and 5,879,828 describe membrane electrode assemblies which are prepared with use of vacuum deposition of metallic catalyst onto whisker-like supports held by a substrate. The supported catalyst is then transferred from the substrate to the membrane or electrode during assembly of the membrane electrode assembly. These patents, however, do not teach or suggest catalyst in a form which is not intimately joined or bonded to the whisker support, which is generally non-conductive. Also, these patents teach that use of ionically conductive polymer in the electrode is undesirable, and that maximum contact between the catalyst and the ionically conductive material is not important. Growth of the catalyst appears to be organized as opposed to random. It would desirable to prepare structures which consist essentially of elements which do not include and do not require use of whisker-like structures to support the catalyst, and which have extensive contact between catalyst and tonically conductive material. Also, random growth of catalyst structure can be important.
In general, therefore, the prior art apparently does not teach, demonstrate, or even suggest fuel cell technology which is suitable for the current or next generation commercial demands.